Planning for the weather is an important factor for making your trip to Siantar a success, especially when you're splurging. The hottest months are usually June and July, with an average temperature of 24°C, while the coldest months are January and February, with an average of 23°C. The rainiest months in Siantar are November, December, October and September, with each month seeing an average of 421 mm of rainfall.